CHICAGO AREA CLIMATOLOGY

Tom SkillingCHICAGO TRIBUNE

Nearly 20 percent of all significant Chicago-area tornadoes occur in the fall. The National Weather Service has compiled statistics on "significant tornadoes" (F2 wind speed or greater, which is 113 m.p.h. or higher) that have occurred in Cook and seven surrounding counties dating back to 1855. Of the 86 significant tornadoes that have been tabulated, slightly more than half have occurred in the spring (March-May) and close to 30 percent in the summer (June-August). None have been noted during the winter months. Albeit small, there seems to be a secondary late-season maxima in November. In fact, there have been as many F2 or greater tornadoes in November (5) as in July. Another interesting statistic is the frequency of significant tornado outbreaks, defined as two or more significant tornadoes on the same day. Of the 14 such "outbreaks" since 1855, three have occurred in the fall and two in November--only April (4) and May (3) have had more. Fortunately, tornado-related fatalities and injuries have been much lower in the fall. This season's portion of significant tornadoes has produced only three of 81 deaths (4 percent) and 149 of 1,558 injuries (9.5 percent).

Source: National Weather Service

WGN-TV/Paul Dailey

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Tom Skilling is chief meteorologist at WGN-TV. His forecasts can be seen Monday through Friday on WGN-TV News at noon and 9 p.m.